Satellites and other Space vehicles customarily serve as a platform for electronic communication and detection equipment in outer space. Through antennas associated with that equipment a communications link is established to transmit and/or receive radio signals to remote locations, including locations on the Earth. Solar arrays are also conventionally deployed aboard space craft to convert the sun's radiant energy into electrical power used to charge the on-board DC batteries that furnish electrical power to that communication equipment.
The solar arrays and antenna reflectors and feed assemblies are referred to as "deployables". That is, for lift off into space, they are physically condensed or folded into the smallest possible package to better use the limited space available on the space craft. However, once the space craft attains orbit, the communications antennas and solar arrays are unfolded and held in position alongside the exterior of the satellite to serve their respective communication and power functions. To do so the antenna reflector and feed and the solar arrays are often supported or held by a support arm or bracket, sometimes referred to as a boom. The support arm is also designed to occupy minimum storage space for lift off and transition into orbit and thereafter extend to the arm's full reach.
One such support arm is a telescoping tube structure, variations of which may be known to the lay reader. Such a tube arrangement is conventionally found in the automobile antenna and in the shaft of collapsible umbrellas, a structure likely familiar to most readers. In that structure a number of cylindrical tubes of consecutively smaller diameter are stacked or "nested" concentrically inside one another. The tube structure may be extended to its full length by arranging the tubes along a common axis. Adjacent tubes in the assembly are linked at a joint, whereby pulling out the end tube engages the next tube at the joint which, with continued pulling, is also withdrawn and axially extended. That process continues until all of the tubes that were confined in the largest cylinder have been withdrawn.
By extending the smallest diameter tube to full length, the end of that tube links to the top end of the next tube in the order, and, with continued pulling that next tube is also axially withdrawn from the larger tube until its end links to the front end of the third tube in the order. And with continued pulling on the first tube, the third tube is also withdrawn. This action continues, with the rear end of the withdrawn tube engaging and forming a joint with the top end of the next tube in the bundle, until all the tubes are withdrawn and arrayed end to end to form a single long tube. That long tube is stabilized by the formed joints. It is appreciated that the order of tube extension may be reversed, ie. The largest diameter tube deploying first; or may be random. At full extension, all joints engage and stabilize the structure.
At first impression, such a telescoping tube arrangement appears to offer an attractive solution for deploying spacecraft antenna components and solar arrays in outer space. With automobile antennas and umbrella shafts, however, the frictional contact inherent in the tube to tube link aids in maintaining the tubes in the linearly extended relationship. For spacecraft application, that high friction relationship is unacceptable. In a telescoping tube for spacecraft application, the friction or, as variously termed, "drag", between the tubes when the tubes are extending, should be very low. And, when finally latched at the end of travel, the tubes should be positively and accurately located and provide a very stiff joint so the support arm formed thereby does not unduly vibrate, wobble or sway. Such features are unavailable with the telescoping tubes employed in automobile antennas or collapsible antennas.
The diameter of the tubes forming a telescoping boom affects the bending and torsional stiffness of that boom. The smaller diameter, the better. When several tubes are telescoped, one into the other, the tube diameters of each successive tube must be such as to allow the successive tube to fit inside the adjacent tube, and to clear the joint. Constructions using thick-walled tubes necessarily result in larger diameter tubes, than telescoping thin-walled tubes. By minimizing the thickness of the tubes to the greatest possible extent, the diameter of the nested tubes can be minimized and the most advantageous stiffness characteristic achieved.
Accordingly an object of the present invention is to provide a new mechanical latching joint for connecting tubes in a telescoping tube structure.
Another object of the invention is to provide a telescoping tube arrangement in which the tubes may be extended with minimal force and, once extended, exhibit such joint stiffness as required to accurately position the end of the tube assembly , and stabilize the segments.
And an ancillary object of the invention is to minimize the thickness of the joint wall in a telescoping tube structure, thereby requiring minimum "stepping down" in adjacent tubes.